1. Field of the Invention
The present invention relates to an automatic gain control circuit used in processing of a video signal.
2. Description of the Related Art
In a video signal processing device which displays a video signal of NTSC system (composite video signal) as a video image, there is used an automatic gain control circuit which amplifies a video signal to a certain level in an analog-to-digital converter in order to maintain a dynamic range of the brightness of the video image approximately constant.
As shown in FIGS. 3 and 4, a video signal SIN of one field includes a vertical synchronization period TV, a horizontal blanking period TH, and a video signal period TS. The video signal SIN of one horizontal line includes a horizontal blanking period TH, a color burst period TC, and an effective video signal period TI. In FIGS. 3 and 4, in order to simplify explanation, the video signal SIN is displayed with the ratios among periods of the video signal SIN differing from those of the actual video signal SIN.
As shown in FIG. 5, an automatic gain control circuit 100 of related art comprises an analog-to-digital converter (A/D converter) 10, a front end block 12, an automatic gain control circuit (AGC circuit) 14, and a pulse width modulation circuit (PWM circuit) 16. A video signal which is input to the A/D converter 10 is converted from an analog signal to a digital signal. In this process, according to a width of a pulse which is input from the PWM circuit 16, the dynamic range of the digital conversion is adjusted. The digitized video signal is output to the front end block 12. At the front end block 12, a pre-process for the video signal is executed. The pre-process includes processes such as, for example, filtering and Γ correction. The video signal passing through the front end block 12 is input to the AGC circuit 14.
The AGC circuit 14 extracts a signal level of the horizontal blanking period TH of the video signal and outputs the signal level to the PWM circuit 16. The PWM circuit 16 generates a pulse signal having a pulse width which is proportional to the input signal level, and outputs the pulse signal to the R/D converter 10. As shown in FIG. 6A, the A/D converter 10 adjusts the dynamic range according to the pulse width of the pulse signal and digitizes the video signal. In this manner, the level of the video signal is fed back to the A/D converter 10, and the dynamic range of the video signal is maintained approximately constant. Such adjustment of the automatic gain control is executed in an initial adjustment process performed after a manufacturing process of the video signal processing device.
However, as shown in FIG. 6B, the signal level in the horizontal blanking period TH of the video signal may vary due to an influence of noise or the like. In this case, if the dynamic range of the A/D converter 10 is determined by reference to the signal level of the horizontal blanking period TH, the dynamic range of the video signal cannot be appropriately adjusted.